#include <bits/stdc++.h>

#define pb push_back
#define mp make_pair
#define mt make_tuple
#define ll long long
#define pii pair<int,int>
#define tii tuple <int,int,int>
#define N 100005
#define mod 2000003
#define X first
#define Y second
#define eps 0.0000000001
#define all(x) x.begin(),x.end()
#define tot(x) x+1,x+n+1
using namespace std;
map<string, bool>este;
int n, i;
string s[110], sol, t;
bool ok(string s) {
    if(s == "")
        return 1;

    if(s.size() == 0)
        return 1;

    int n = s.size();
    bool ret = 0;

    for(int i = 0; i < n  && !ret; i++) {
        t = s.substr(0, i + 1);
        string w = s.substr(i + 1, n);

        if(este[t])
            ret |= ok(w);
    }

    return ret;
}
int main() {
    cin.sync_with_stdio(0);
    cout.sync_with_stdio(0);
    cin >> n;

    for(i = 1; i <= n; i++) {
        cin >> s[i];
        este[s[i]] = 1;
    }

    for(i = 1; i <= n; i++) {
        este[s[i]] = 0;

        if(ok(s[i])) {
            if(s[i].size() > sol.size())
                sol = s[i];
        }

        este[s[i]] = 1;
    }

    if(sol == "")
        cout << -1;
    else
        cout << sol;

    return 0;
}